Most species are not driven
to extinction before genetic
factors impact them
Kristin Debord
Shaun Fike
Two contrasting view points


Genetic factors do not affect
extinction rates.
Genetic factors do affect extinctions
rates
Factors Affecting Species
Population Size
Habitat loss
 Over exploitation
 Impact of introduced species
 pollution

Stochastic Factors
Demographic
 Environmental
 Genetic stochastic factors
 Natural catastrophes

Accumulation of inbreeding
Hg/H0 = [1-1/2(2Ne)]g = 1-F
Hg = heterozygosity @ generation g
 H0= initial heterozygosity
 Ne= long term effective population
size
 F= inbreeding coefficient

Methods

Conducted metaanalysis using IUCNthe World Conservation Union Red
List threatened categorization
system
Methods cont.
Analyses were done on percentage
differences in heterozygosity
between threatened and the nearest
related nonthreatened species or
group of species.
 Analyses based on data for
allozymes, microsatellites, and
minisatellites.

Methods cont.



Data from listed species were paired with
data of the same type from the most
closely related species.
Used observed or expected heterozygosity
If allozyme and microsatellite were
available for the same taxon, the
combined weighted average was used.
Methods cont. (Major Taxa)


Plants were subdivided in to
gymnosperms and angiosperms.
Animals were divided into
invertebrates and vertebrates which
were further subdivided into
poikilotherms, homeotherms, birds
and mammals.
Null Hypothesis

Genetic diversity of threatened taxa
does not differ from that of non
threatened taxa.
Alternative Hypothesis

Threatened taxa have less genetic
diversity than comparable non
threatened taxa.
Data Analyses


The defining measure used was the
percentage difference in
heterozygosity.
(100*(nonthreatened – threatened)/
nonthreatened) or (100 *
(nonthreatened- threatened)/
threatened)
Data analyses cont.

Wilcoxon’s signed ranked tests were
performed on the difference in
heterozygosity of each threatened
taxon compared with the most
closely related available taxon or
taxa not included in the IUCN Red
Lists.
Data analyses cont.

Tested for differences among the
different Red List categories using
Kruskal-Wallis tests.
Results

77% of the 170 threatened taxa had
lower heterozygosity than related
nonthreatened taxa.
Results cont.
Results cont.
Discussion
A majority of the threatened taxa
showed lower genetic diversity than
related nonthreatened species.
 The mean difference was found to be
35% and the median difference was
found to be 40%.

Discussion cont.
Taxa that do not currently show any
genetic impact may still experience
genetic impact prior to extinction.
 Vulnerable taxa show a 10%
probability of extinction within 100
years.

Discussion cont.
It is unclear whether genetic factors
have contributed to the current
threatened status of taxa in the
study.
 Reduced genetic diversity is a marker
of reduced reproductive fitness,
meaning that the extinction risk is
elevated.

Links Between Reduced Genetic
Diversity and Extinction Risk
1.
2.
3.
Reduced genetic diversity has been
shown to reduce times to extinction.
The difference in heterozygosity is a
measure of the inbreeding coefficient of
a taxon.
Computer projections show inbreeding
depression adversely affects extinction
risk even when all other factors are
operating.
When are genetic effects of lowered
diversity of sufficient magnitude that
they must be managed?
Depends on inbreeding coefficient,
effective population size, and number
of generations
 It is also likely to vary among species
in relation to population growth rate

“No Genetic Impact” Doesn’t Apply
to Most Threatened Taxa
1.
2.
3.
4.
Ratios of effective population sizes are
lower than were assumed in 1988.
Impacts of interactions between genetic
and other stochastic factors may have
been underestimated.
Impacts of inbreeding depression were
likely underestimated.
Overestimated effectiveness of natural
selection.
Conclusion
Most threatened taxa have lower
genetic diversity than closely related
nonthreatened taxa.
 Most species are not driven to
extinction before genetic factors
impact them.
